Volume control circuits



mms

may 18, 1943 J. F. CROWLEY VOLUME CONTROL CIRCUITS.

Filed May 8, 1957 4ND OUTPUT S YSTEM m M a M m M POS/T/U/YS 0F WLUME NTR L Patented May 18, 1943 VOLUME CONTROL CIRCUITS John F. Crowley, Philadelphia, Pa., assignor, by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application May 8, 1937, Serial No. 141,536

6 Claims.

This invention relates to volume control circuits for radio receivers or the like and, more particularly, to a novel volume control circuit of the type employing a degenerative feed-back connection in the audio amplifier of a radio receiver.

Prior devices of thisgeneral class have been characteristically deficient in that they have lacked adequate and satisfactory means for controlling the output volume level of the system. More specifically, such devices have been characterized by the fact that the minimum output volume level obtainable has been too high and it has been impossible in the employment of such devices to reduce the output volume level to a desired minimum value.

The present invention has for its principal object the provision of an improved volume control circuit which is free of the objectionable characteristics of prior devices and which embodies the provision of simple and effective means for controlling the output volume level in the desired manner.

Another object of the invention. is to provide a novel degenerative feed-back connection in a device of this class whereby the defects of prior devices are substantially eliminated.

A further object of the invention is to provide a novel volume control device for controlling both a degenerative feed-back signal and an audio signal, both of which are applied to the audio channel.

Other objects and features of the invention will appear hereinafter.

In the accompanying drawing:

Fig. 1 is a schematic illustration of a portion of a radio receiver embodying the invention; and

Fig. 2 is a graph illustrating certain desirable characteristics of the invention.

Referring to Fig. 1, there is shown a source of modulated carrier signal Ec which might comprise the last intermediate frequency or radio frequency stage of a radio receiver. This signal is supplied to the diode rectifier D by means of the tuned transformer T. The diode rectifier D may comprise part of a diode-triode tube V, which tube serves both as a detector and as an audio amplifier, as well understood. The diode rectifier serves to demodulate the incoming modulated carrier signal, and the audio signal which is thus obtained flows through the diode circuit which includes the audio load impedance R and the carrier frequency by-pass condenser C. There is thus built up across the resistance R an audio signal voltage which is transferred to the grid-cathode or input circuit of the amplifier portion of the tube V by means of the filter circuit including condenser C1, resistance R1, bypass condenser C2, and the volume control device or potentiometer P. The condensers C and C: may be of such capacity as to have low impedance at the carrier frequency but very high impedance at the audio frequency. The condenser C1 should have such capacity that it is adapted to. transmit the audio signal without attenuation and serves only as a blocking condenser.

It will be seen, therefore, that the audio signal formed in the diode circuit will be transmitted to the potentiometer P and will build up an audio signal voltage thereacross. A desired portion of this signal is transferred to the control grid of the tube V by means of the movable arm of the potentiometer, the blocking condenser Ca and the grid-leak resistance R2. The output signal from tube V is supplied to the usual audio amplifier and output system which, in turn, supplies the amplified audio signal to the loud speaker LS or other utilization means. Degeneration is obtained by feeding back a portion of the amplifier output signal to the input circuit of the amplifier portion of tube V, This may be done conveniently by grounding one side of the voice coil of the loud speaker and by connecting the other side of the voice coil through a condenser C4 and a resistance R3 to a predetermined tap point 3 on the potentiometer as clearly illustrated. It will be noted that the tap point 3 is also connected to ground through a resistor R4, the purpose of which is to permit the use of standard potentiometers such as are now commercially available. These potentiometers have a high and somewhat uncertain resistance whereas the present invention can best be practiced employing lower resistances whose relative values can be controlled more readily. By shunting the tapped portion of the volume control device or potentiometer with a low resistance of the wirewound type for example, the design of the filter circuits which may be employed in the feed-back circuit can be better controlled and parts of convenient sizes may be employed. This enables the use of standard high impedance potentiometers, since such variations in the resistance thereof as obtain in commercial practice have no deleterious effect upon the circuit. It will be understood that the feed-back signal may be ob tained from other parts of the audio amplifier or output circuit.

By means of the feed-back connection, there is established between the tap point 3 and ground a signal determined by the signal in the output circuit of the system; This feed-back signal should be of opposite phase with respect to the signal supplied by the diode D. In other words, the feed-back circuit should be degeneratively connected. Broadly speaking, degenerative circuits are old in the art and require little explanation. It suffices to point out that such a circuit operates first to reduce the over-all gain of the system; second, to reduce materially the amount of distortion of the output signal, and third, to enable various forms of tone control by use of suitable filter circuits. In the circuit illustrated, for example, if the condenser C4 is designed so as to have an impedance comparable to that of the resistance R3 at low audio frequencies-the circuit will operate as a bass compensation device and will increase the bass response, since the amount of signal fed back and, consequently, the amount of degeneration, will be reduced at the low frequencies. At higher frequencies, however, the impedance of the condenser C4 will become negligible and the amount of signal fed back will then be limited by the ratio of the resistances.

In analyzing circuits of the type here involved, it is convenient to consider them in terms of the amount of the degeneration employed and to speak in terms of the degeneration ratio. Such instance. The ratio may be measured by supplying a standard modulated input signal to the system and then measuring the output signals obtained, first, with the feed-back connection to the condenser C4 opened and with the condenser grounded and, second, with the feed-back connection closed as illustrated. The ratio of the two output signals thus obtained will be the degeneration ratio.

In Fig. 2, there are shown the degeneration ratio and output volume level characteristics of the device of the invention. The graphs or curves A and B represent, respectively, the degeneration ratio and the output volume level for various positions of the potentiometer arm between the points I and 2. arrows in Fig. 1, the potentiometer arm is adapted to move between the points I and 2. The three principal positions I, 2 and 3 of the potentiometer arm are indicated in Fig. 2. The horizontal broken-line represents a degeneration ratio of one.

As shown by the graph A, the degeneration ratio starts at some value greater than one when the potentiometer arm is on point I and, as the arm is moved toward point 2, the degeneration ratio increases to a maximum or peak at the tapped point 3 and then decreases to one as the potentiometer arm is moved from point 3 to point 2. As shown by curve B, the output volume level which is obtained when a standard signal is supplied to the diode D, is a maximum when the potentiometer arm is on point I and decreases to a low value as the arm is moved to point 3. The volume level then decreases to zero as the potentiometer arm is moved from point 3 to point 2.

In prior devices of this general class, the end of the volume control resistance has been connected to the feed-back circuit, and a satisfactory zero volume level could not be obtained As indicated by the employing such devices. By connecting the feedback circuit to a tapped point on the volume control resistance according to the present invention, complete control of the volume level is had and a satisfactory minimum volume level may be obtained. It is noteworthy also that prior devices have generally had a minimum output level somewhat greater than the output level obtained by the present invention at the tapped point I. It will be apparent, therefore, that the present invention enables better control and more desirable characteristics .of the volume level than prior devices of the same general class.

Variation in the degeneration ratio as illustrated by graph A in Fig. 2 obtains by variation in the amount of signal feed-back compared to the effective portion of the signal supplied by the diode D, as the position of the potentiometer arm is varied. When the arm is at maximum volume point I, nearly all of the audio signal supplied by the diode is transferred to the input circuit of the amplifier portion of tube V. This signal is opposed, however, by the feed-back signal which is somewhat attenuated, the amount of attenuation being determined by the internal audio impedance of the diode circuit and the impedance of the potentiometer resistance between the points I and 3. As the potentiometer arm is moved toward the point 3, the portion of the signal from the diode circuit which is supplied to the input circuit of the amplifier, is decreased and, at the same time, the attenuation of the feed-back signal is decreased, thus increasing the degeneration ratio as shown by graph A. Since the gain of the amplifier is inversely pro- .portional to the amount of degeneration, in

order to obtain the advantages of low distortion and greater stability, it is desirable that the amount of degeneration be small for maximum output and that the degeneration increase for lower output levels. At lower output levels, the gain, of course, is no longer important. As the potentiometer arm is moved from point 3 to point 2, the amount of signal transferred from the diode circuit is still further reduced, and since the end of the potentiometer resistance is connected through ground to the cathode of the diode for audio signals, the signal transfer may actually be reduced to zero when the potentiometerarm is at the point 2. At'the same time, the feed-back signal is increasingly attenuated as the potentiometer arm moves from point 3 to point 2 but, since the output level is very low at this time, the amount of degeneration is no longer of great importance.

The tap point 3 of the potentiometer should preferably be as near the bottom as is commercially practical. The resistance between the tap point 3 and the ground point 2 may be any convenient value greater than that of the resistor R4. In one satisfactory embodiment of the invention, the tap point occurred at twenty percent (20%) of the total rotation of the volume control device, and the resistance between the tap point and ground was 40,000 ohms. The resistance of the total potentiometer was 1 megohm. The values for other parts of the circuit were as follows: R=0.33 megohm, R1=0.1 megohm, Rz=-1.5 megohms, Rz=8,000 ohms, R4=3,000 ohms, 0:110 mmfs., C1=0.004 mf., C2=110 mmfs., C4=0.05 mf.

A type '75 diode triode amplifier tube was employed with a conventional diode transformer. The output impedance of the voice coil of the loud speaker was of the order of magnitude of 2 or 3 ohms. In this particular embodiment of the invention, the degeneration ratio measured at 400 C. P. S. at the maximum volume point was of the order of magnitude of 1.5 and reached a peak at the tapped point of the order of magnitude of 3. It will be understood, of course, that these values may be varied over wide limits and are not to be considered as limiting the invention.

Although a specific preferred form of the invention has been illustrated and described for the purpose of disclosure, it will be understood that the invention is not limited thereto but is capable of various embodiments and modifications. The scope of the invention is defined only by the appended claims.

I- claim:

1. In a radio receiver, a source of audio signals, an audio amplifier having an input circuit and an output circuit, a volume control device connected to said source and connecting said input circuit to said output circuit degeneratively for transferring to said input circuit the difierence between a portion of the signal in said output circuit and a portion of the signal from said source, said degeneratively connected volume control device including means for varying the said portion of each of said signals in such manner that the degenerative ratio is approximately one when the volume control is adjusted for minimum output level, and said ratio increases to a maximum and then decreases as the volume control is progressively adjusted for higher output levels.

2. In a radio receiver, a source of audio signals, an audio amplifier having an input circuit and an output circuit, a volume control device connected to said source and connecting said input circuit to said output circuit degeneratively for transferring to said input circuit controllable portions of the signal in said output circuit and the signal from said source, said degeneratively connected volume control device including means for varying the said portions of said signals in such manner that the degenerative signal increases from a relatively low value to a maximum value as the volume control is progressively adjusted to decrease the output ievel from a maximum value to a low value, and the degenerative signal decreases as the volume control is adjusted to decrease the output level further to a minimum value.

3. In a radio receiver, a source of audio signals, an audio amplifier having an input circuit and an output circuit, a volume control device connected to said source and connecting said input circuit to said output circuit degeneratively for transferring to said input circuit controllable portions of the signal in said output circuit and the signal from said source, said degeneratively connected volume control device including means for varying the said portions of said signals in inverse relation to one another over a portion of the adjustment range of said volume control device, and for varying the said portions of said signals in direct relation to one another overanother portion of the adjustment range of said volume control device.

91. In an audio amplifying system having input and output circuits, a volume control potentiometer connected to said input circuit to supply a variable portion of an input signal thereto, means connecting the low potential end or said potentiometer to one side of said output circuit, impedance means connected across said output circuit, and means connecting an intermediate point of said impedance means to an intermediate point of said potentiometer.

5. In an audio amplifying system having input and output circuits, a volume control potentiometer connected to said input circuit to sup ply a variable portion of an input signal thereto, impedance means connected across said output circuit, and means connecting a portion of said impedance means in shunt with a portion of said potentiometer.

, 6. In an audio amplifying system having input and output circuits, a volume control potentiometer connected to said input circuit to $119 ply a variable portion of an input signal thereto, frequency selective impedance means connected across said output circuit, and means connecting a portion of said impedance means in shunt with a portion of said potentiometer. 

